In order to improve reservoir fluid recognition, the sensitivity of array resistivity response to the difference of the invasion property in both oil-bearing layer and water layer is analyzed. Then the primary inversion is carried out based on the array resistivity log. Differing from the monotonic resistive invasion profile of water layer, the radial resistivity of fresh mud-invaded oil-bearing layer presents complex distribution characteristics, such as nonlinear increase, increasing to decreasing and low resistivity annulus. Under certain conditions, array resistivity log can reflect these changes and the array induction log is more sensitive. Nevertheless, due to the effect of factors like large invasion depth, reservoir physical property and oiliness, the resistivity can exhibit great deviation from the actual mud filtrate invasion profile. We construct a five-parameter formation model to simulate the complex resistivity distribution of fresh mud-invaded formation. Then, based on the principle of non-linear least squares, the measured array resistivity log is used for inversion with the Marquardt method. It is demonstrated that the inverted resistivity presents a typical nonmonotonic variation in oil-bearing layer and a monotonic variation in water layer, which agrees well with the actual invasion property. This can help to realize efficient reservoir fluid recognition.


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